def find_diff(s, t):
    if not s:
        return t
    table = {i: 0 for i in range(26)}
    for i in range(len(t)):
        if i < len(s):
            table[ord(s[i]) - ord('a')] -= 1
        table[ord(t[i]) - ord('a')] += 1
    for i in range(26):
        if table[i] != 0:
            return chr(i + 97)
    return 'a'


s = 'ae'
t = 'eab'
r = find_diff(s, t)
print(r)
